Play the "X Marks The Spot" level (level 61), which has a single rainbow pyramid in the center of the landscape. Shoot the pyramid. If it does not reappear purple, intentionally lose and try again. You will eventually get a new pit in the center of the "X". Fall into that pit. After that, choose a level with Limeys at the selection screen. Until the game is restarted, the Limeys will have a new appearance.